Fiber optic systems feature fiberoptic splitters or couplers as a means to branch optical power into more than one fiber waveguide. Often it is desired to couple optical power from a common bus fiber into many side branches. Taps are located serially along the length of the bus fiber, each tap providing only a small fraction of the total power carried on the bus for use by attached equipment The optical power budget of such a system typically interrelates with the amount of loss of optical power that can be tolerated at each tap point. Since the power available at each tap point depends upon the total power remaining in the bus fiber at that point, system designs critically depend on the splitting ratio of each coupler being sufficiently constant over the entire optical wavelength band of sources usable in the system. Additionally, the coupling ratio must be sufficiently constant in coupling ratio with respect to changes in temperature, physical stress and input optical polarization.
Typically, conventional couplers made by the familiar technique of fused tapering, exhibit acceptably low loss, but the coupling ratio depends on the wavelength of light passing through the coupler. Since the wavelength of optical sources can vary over considerable ranges, and since it often is necessary to use more than one optical wavelength in a system, it is highly desirable to use couplers which exhibit reduced sensitivity of coupling ratio to optical wavelength. Couplers having a nominally constant coupling ratio over the wavelength band of interest are referred to as wideband couplers, wavelength flattened couplers, wavelength independent couplers, broad-band couplers, etc. By using couplers that have a sufficient constancy over a sufficient band width, with acceptable excess loss, and sufficient constancy in performance with changes in the temperature and stress environment, systems can be provided for working over a design range of wavelengths and environmental conditions. It follows that improvement in the constancy of coupling ratio while keeping excess loss within acceptable limits can lead to important improvement in the performance and lower cost for many optical systems.
Heretofore, wideband couplers have been made by preselection of the difference in propagation constant of the constituent fibers relative to the desired coupling ratio. In this manner the first maximum, a relatively flat portion of the coupling ratio curve that oscillates during drawing of the coupler, can be made to coincide with the coupling ratio desired. The coupling performance of the resulting coupler can be relatively constant due to operating with the flat portion of the coupling ratio curve that occurs at maxima. Fibers having such preselected, different propagation constants, e.g., can be fused together in a manner to achieve identical coupling at two wavelengths by selection of the stopping point during fusing/drawing of the fibers, either by observing the varying coupling ratios or selecting manufacturing parameters that have been predetermined to produce the desired ratio. This method is most useful in a two wavelength system where both wavelengths are well known in advance. One of the fibers of different propagation constants can e.g. be powered by the two wavelengths while the fibers are fused by thermal drawing. Drawing is stopped as soon as the desired coupling ratio at wavelengths is observed. The coupling ratio in such cases is the result of the amount and nature (constructive) of the interference between the symmetric and antisymmetric modes in each of which substantial energy is propagating in the coupler. For this discussion we shall refer to couplers made from fibers of different propagation constant drawn to an early occurrence of the desired coupling ratio as "short-draw Delta-B" type wideband couplers.
A difference in propagation constant of two fibers is acquired in many ways. In one approach identical fibers are processed to have different diameters, e.g., by drawing one fiber into a tapered section of reduced diameter relative to the other fiber. The tapered section is then fused with an unprocessed fiber or with a fiber that was tapered more or less than the first fiber. Different propagation constants can also be obtained by etching one or both fibers so that their diameters are different before fusion or by selecting fibers with different V numbers.
Couplers made in this manner, whose coupling ratio is dependent upon interference between symmetric and antisymmetric modes, typically demonstrate fairly low excess losses but often exhibit coupling ratio variations of +/-9% or more over about a 300 nanometer optical wavelength range. This degree of coupling variation can account for significant cumulative errors in system power budget when many such couplers are used as serial taps.
As a variation of the short-draw Delta-B method, wavelength response flattening has also been achieved by fusing fibers that have different core refractive indices. Again, the desired coupling ratio is dependent upon the degree of interference between the symmetric and antisymmetric modes in which the energy of the coupler propagates. The utility of such a coupler may in some cases be limited by the presence of dissimilar glasses. E.g., splicing of fibers with differing indices into a system bus fiber creates cumulative splicing loss variations that may prove intolerable to system designers. Additionally, in manufacturing, quality control of two different fibers may be more costly than control of a single type of fiber.
In an experiment that has been reported, wavelength insensitivity has been obtained by drawing two identical fibers over a much greater distance than used in other approaches, until a different phenomenon occurs (see Bilodeau et al., "Compact, low loss, fused biconical taper couplers: overcoupled operation and antisymmetric supermode cut off," Optic Letters, Vol. 12, No. 8, 1987). Herein this method is referred to as the "extended-draw" method. As the fibers are fused and progressively drawn, the coupling between the two fibers oscillates. This is a common observation to anyone skilled in the art of coupler fabrication. (see also Bilodeau et al, Fabrication Technique for Low-Loss Fused Taper Directional Couplers and Pressure Sensor Produced Thereby, U.S. Pat. No. 4,895,423, Jan., 1990.) In this latter reference it is reported that by halting the drawing of the the coupler after several hundred to several thousand cycles of the coupling ratio, the coupler becomes increasingly sensitive to perturbations of the optical media, e.g., by temperature or pressure variations at the coupling region. A coupler operating in the region of rapid coupling oscillation relative to draw length experiences increasingly sensitive interference between the symmetric and antisymmetric modes guided by the coupling region and becomes a suitable means for sensing environmental phenomena but certainly not suitable for communication network applications.
However, as reported in the former reference, if the drawing process is continued long enough to cause oscillation through many cycles, eventually the variation of coupling ratio has been found to cease and the coupling ratio stabilizes at about 50%, attributed to cut-off of the antisymmetric mode of energy propagation. The coupling ratio obtained then typically varies within +/-2% over the wavelength range, an attractive characteristic.
However, using the extended-draw method reported, optical power loss of about 50% was observed by the authors. Even if such excess loss might be tolerable in a few applications, the majority of communications systems designs cannot of course tolerate the repeated loss of half the system optical power at each splitting point.
As another part of the background of the present invention, in many applications, it is often necessary to branch one optical fiber into more than two outputs. Couplers which perform this branching are often referred to as star couplers. Star couplers can be made by fusing more than two fibers in a common fused region. A star coupler of this type intrinsically has an equal number of input and output fibers. In a single optical wavelength system application, typically only one of the fibers is used as the input. If more than one wavelength is used in the system, each separate wavelength may be input on a different optical fiber. The action of the coupler is to split the sum of optical power in each input fiber into more or less equal fractions of the input sum in each output fiber. E.g., if three optical inputs P1, P2, P3 are used in a 3.times.3 star coupler, each output fiber might carry output power equal to (P1+P2+P3)/3, neglecting excess loss. In general, star couplers are designated as N.times.N or 1.times.N couplers, N denoting the number of fused fibers.
To make a star coupler, a preferred method of prior teachings disposes six fibers around a central fiber. All seven of these fibers have substantially identical diameters. So doing provides optimum coupling uniformity. Such a coupler is drawn until the coupling ratios, as measured by the amounts of optical power carried in each output fiber, is essentially equal among all the fibers. This method can yield good coupling uniformity, low excess loss, and a degree of wideband behavior based on the principles noted above with regard to short draw couplers. The method is however limited to the fabrication of 1.times.6 and 1.times.7 port couplers. Most systems designs require 1.times.4 and 1.times.8 port configurations.
Another method of performing branching into many fibers requires the fabrication of a tree structure using a collection of 2.times.2 couplers. The two outputs of a first 2.times.2 coupler are spliced to the input fibers of two more 2.times.2 couplers. The four outputs of these two couplers are spliced to the inputs of four more 2.times.2 couplers. Thus, a tree of seven 2.times.2 couplers provides two inputs and eight outputs. The fabrication of tree structures is, in principle simple, but coupling ratio variations among the couplers and splicing losses between couplers accumulate to cause wide variations in the fraction of input light present in each output. Couplers chosen for tree fabrication must be exceptionally accurate and stable, and the splicing procedure must be quite precise and repeatable.
Considering the cumulative optical power loss in a tree coupler, using extended-draw wideband couplers for such fabrication is not realistic. Alternatively, since the wavelength dependent coupling ratio variation of short-draw Delta-B couplers ranges around+/-9%, very careful selection of short-draw Delta-B couplers is required if these couplers are to be used in a tree and the results are often not as good as desired.
In systems designs requiring star type couplers three parameters characterizing the couplers are important: 1) the "uniformity" of the coupling ratio describes the degree to which each of the N outputs carries essentially 1/N of the total output power; 2) the constancy of the coupling ratio with respect to wavelength; and, 3) the excess loss. Prior art has emphasized the importance of uniformity. In applications requiring equal splits of optical power, e.g., a bus which is split into eight equally tapped branches, uniformity is a critical parameter.
On the other hand, many distribution systems need to split the optical power from a central source into many legs of unequal length, or into legs which will service different numbers of serial user taps. In this case it is wasteful of optical power to split the bus uniformly. The legs which support fewer serial taps require less optical power than those that support many taps. What is needed, therefore is a wideband star coupler providing a selection of splitting ratios appropriate to serve a range of branch tap reguirements. In this case, the uniformity is not critical, but the constancy of coupling to each output with respect to wavelength and environmental conditions is critical. Excess loss may or may not be of critical importance depending on the system reguirements.
The prior art has not shown a favorable way to produce 2.times.2 couplers which possess the combined features of coupling any amount of optical power from 0 to 30%, low loss, and truly flat response over the 1200 to 1600 nm range of typical optical power sources. Similarly, prior art has not shown an economical method to fabricate wideband star couplers in 1.times.4 and 1.times.8 port configurations. Further, prior art has not addressed the need for non-uniform wideband couplers having 1.times.N and N.times.N port configurations.
The present invention addresses the various needs described above and provides improved, practical fiber optic couplers, splitters and similar devices.